Addressing Exam Questions

• C Router (1)
• The only correct answer is C. The correct answer is router.
• A is not correct because a hub sends data to every device on the network
• B is not correct because a network interface card connects a device to a network
• D is not correct because a switch forwards data within a single network using MAC addresses

• C 32 (1)
• The only correct answer is C. The correct answer is 32.
• A is not correct because 4 is the number of groups of numbers in an IPv4 address
• B is not correct because 8 is the number of bits in each group of an IPv4 address
• D is not correct because 128 is the number of bits in an IPv6 address

• B The destination IP address (1)
• The only correct answer is B. The correct answer is the destination IP address.
• A is not correct because a domain name is converted to an IP address by a DNS server before routing begins
• C is not correct because the sender's MAC address is only used to deliver the packet on the local network
• D is not correct because the file name is not used to route packets

• B 192.168.1.1 (1)
• The only correct answer is B. The correct answer is 192.168.1.1.
• A is not correct because it is an incomplete address with only three numbers, not four
• C is not correct because it is an address with five numbers, not four
• D is not correct because it contains 256, which is outside the range 0 to 255

Award one mark for:

• (Destination) IP address (1)

Allow IP address on its own (the destination is implied).

Do not accept source IP address/IP address of the sender.

Do not accept MAC address.

Do not accept domain name/web address.

Award one mark for:

• 0 to 255 (1)

Allow 0–255 / 0-255 / between 0 and 255 / 0 and 255.

Do not accept 1 to 255 (0 is a valid value).

Do not accept any range that includes 256, e.g. 0 to 256.

Award one mark for:

• (An IP address that) can change / is not permanent (1)

Allow a changing IP address.

Allow a temporary IP address.

Allow it is assigned each time the device connects to the internet.

Do not accept an IP address that is assigned (automatically) by a router/DHCP without any idea that it can change.

Do not accept static IP address / fixed IP address / an IP address that never changes (this is a static IP address).

Award one mark for any of the following up to a maximum of two marks:

• A unique identifier/number (1)
• Given to a device on a network/the internet (1)
• Used to send/receive/route data (to the correct destination) (1)

Allow computer/laptop/phone/printer etc. for device.

Do not accept a website address/domain name.

Do not accept MAC address (this is a hardware identifier set by the manufacturer, not an address given to a device for use on a network).

Do not accept 'an address'/'a location' on its own, without saying that it identifies a device on a network/the internet.

Award up to two marks for a linked description such as:

• Identifies a device on a network (1) so data can be sent to the correct device (1)
• Gives a device a unique address (1) so packets can be routed to it across the internet (1)
• Identifies a device on the internet (1) so it can send/receive data to/from other devices (1)

Allow 'unique' only where it is linked to identifying the device or to routing data; 'unique' on its own is not creditworthy.

Do not accept answers that only say it connects a device to the internet.

Do not accept a MAC address as the purpose of an IP address.

Do not accept domain names/web addresses (e.g. google.co.uk) as an IP address.

Award up to two marks for a linked description such as:

• The IP address belongs to the network/router the device is connected to (1) so a different network/router gives it a new address (1)
• Home routers may use dynamic IP addresses (1) which can change when the router reconnects (1)
• The ISP assigns the IP address (1) and may give a different one at a new location/connection (1)

Do not accept answers that only say the laptop has moved.

Do not accept the MAC address changes.

Award up to two marks for a linked description such as:

• Reads the destination IP address from the packet (1) to choose the next router/network on the path (1)
• Uses a routing table that holds destination IP addresses/networks with the matching next hop (1) to forward the packet on the best/quickest route (1)
• Forwards packets from one network to another (1) that is closer to the destination IP address (1)
• Checks the destination IP address against its stored routes (1) so the packet is sent out towards the correct network (1)

Allow 'next hop' / 'next router on the route' / 'next network' / 'gateway' as equivalent wording for where the packet is sent next.

Do not accept answers that only say the router sends the packet to the destination.

Do not accept answers that say the router converts/changes the destination IP address into a MAC address and sends the packet straight to the destination, as no routing step is described.

Award up to two marks for a linked explanation such as:

• Data/packets are delivered to the correct/intended device (1) because the destination IP address identifies that device, so a duplicate address would mean packets could be delivered to the wrong device (1)
• Replies are sent back to the device that requested the data (1) because the sender's IP address identifies that device, so a duplicate address would mean the reply could be sent to the wrong device (1)
• The device can be identified separately from other devices (1) so packets are not delivered to a different device that has the same address (1)

For both marks, the expansion must follow/associate with the statement.

Award one mark for a valid point and a second mark only for a linked reason explaining why a duplicate address would cause a problem.

Do not accept 'it is unique'/'it has its own address' as a point, as this repeats the question.

Do not accept 'so it can connect to the internet' or 'so it can be found' on its own without a link to data/packets being delivered to the correct device.

Allow 'data' for 'packets'.

Award up to two marks for a linked explanation such as:

• The router reads/checks the destination (IP) address in the packet (1) so it can forward/send the packet towards that address/destination (1)
• The router looks up/compares the destination (IP) address (1) to choose the route/next network/next router for the packet (1)
• The router uses the destination (IP) address to choose/select the outgoing connection/network (1) so the packet is sent one step closer to its destination (1)

For both marks, the expansion must follow/associate with the statement.

Do not accept a restatement of the question, such as 'the router uses the IP address to forward the packet', without saying how the address is used, e.g. checking the destination address or choosing the route/next network.

Do not accept answers that say the router gives/adds an IP address to the packet, or that the router uses a MAC address instead of the destination IP address.

Award up to two marks for a linked explanation such as:

• IPv4 addresses are 32-bit (1) so there are only about 4 billion of them/not enough for every device (1)
• More devices are connecting to the internet than IPv4 can address (1) so a larger range of addresses is needed (1)
• IPv6 uses 128-bit addresses (1) so far more devices can be given an address (1)

For both marks, the expansion must follow/associate with the statement.

Do not accept 'IPv6 is newer/better/faster/more secure' without a link to the number of addresses available.

Award up to two marks for a linked explanation such as:

• The laptop is connecting to a different network (1) so it needs an address that belongs to/is unique on that network (1)
• The router/ISP on the new network assigns the address (1) because addresses are given out by the network the device is connected to (1)
• The home IP address is only valid on the home network (1) so it cannot be used to send data on another network (1)

For both marks, the expansion must follow/associate with the statement.

Do not accept 'it has a dynamic IP address' on its own, as this repeats the question.

Do not accept answers that say the IP address is stored in the device's hardware/network card, as this describes the MAC address.

Indicative content

IP addresses

• Each device on the internet needs a unique IP address so that it can be identified by other devices.
• The order is split into packets and each packet is given the destination IP address of the bookshop's web server.
• Each packet also carries the sender's IP address so that the web server can send its reply back to the customer's laptop.
• An IPv4 address is made up of four 8-bit numbers separated by dots, for example 72.129.214.16.
• The destination IP address is the same on every packet in the order, so all the packets are sent towards the same web server.

Routers

• A router reads the destination IP address in each packet and forwards the packet towards the web server.
• A router is connected to more than one network, so it passes packets from the customer's home network onto the internet and then towards the network that holds the web server in another country.
• At each step a router chooses a route for the packet, so packets from the same order can take different routes.
• A router only decides the next step for a packet, so no single device needs to know the whole route.
• The web server receives the packets because each one carries its destination IP address, even if the packets take different routes.

Levels

• Level 0 (0 marks): No rewardable content.
• Level 1 (1–2 marks): Basic, independent points are made, showing elements of understanding of key concepts/principles of computer science. (AO1) The discussion will contain basic information with little linkage between points made or application to the context. (AO2)
• Level 2 (3–4 marks): Demonstrates adequate understanding of key concepts/principles of computer science. (AO1) The discussion shows some linkages and lines of reasoning with some structure and application to the context. (AO2)
• Level 3 (5–6 marks): Demonstrates comprehensive understanding of key concepts/principles of computer science to support the discussion being presented. (AO1) The discussion is well developed, with sustained lines of reasoning that are coherent and logically structured, and which clearly apply to the context. (AO2)

Level 3 responses must cover both the role of IP addresses and the role of routers with developed linkage to the order; an answer that covers only one bullet, or gives basic independent points with little linkage, is limited to Level 2 or below.

Do not credit MAC addresses as the address used by routers to forward packets across the internet, or the idea that the laptop sends the order directly to the web server without routers.